Methods for modifying specific regions of the DNA of the oncogenic virus SV-40 will be developed. The effects of the modifications upon the physical and biological properties of the DNAs will be studied. Those treatments which prove to be mutagenic will be employed to isolate mutants of SV-40 which lack viral functions expressed in SV-40 transformed cells. Such mutants will be characterized by biochemical an genetic means to define the functions involved in cell transformation. In addition, cell mutants which can suppress the defective viral functions will be isolated and characterized to define functions within cells which can modify viral expression, or aid in viral transformation.